1. Technical Field
The present invention relates to a vibratory gyroscope and a method for manufacturing the same.
2. Related Arts
It has been studied to mount a vibratory gyroscope in a car body for controlling its orientation. In such application, the vibratory gyroscope is mounted in a housing, which is then attached on a chassis of the car body. External vibration is inevitably transmitted through the chassis to the gyroscope, possibly causing a malfunction or vibration noise. It is necessary to minimize the noise caused by the external vibration.
Japanese patent application publication xe2x80x9cKokaixe2x80x9d 269228/1997 discloses, in a gyroscope using a tuning-fork type vibrator comprising a base and a tuning-fork, a method for reducing the cross-talk noise induced by external vibration. In the column (0002), this publication discloses a method to reduce the vibration noise by adjusting the length of the base within a certain range. J. Yukawa et al. xe2x80x9cAngular Rate Sensor for Dynamic Chassis Controlxe2x80x9d (Sensors and Actuators) 980269, pages 49 to 54, 1998 discloses a method for reducing vibration noise in a vibratory gyroscope comprising a metal vibrator and poly-crystalline piezoelectric elements to drive the vibrator. They reduced vibration noise, due to the beat of the vibrator, by increasing the detuned frequency to a value suitably larger than the cut-off frequency of a low pass filter. The detuned frequency is the difference of resonance frequencies of driving and detection modes of the vibrator. The detuned frequency disclosed is as large as 600 Hz.
The present inventors have researched to reduce the influence of external vibration on vibratory gyroscopes using a vibrator, and subsequently found that, when external vibration of the same frequency as the detuned frequency is transmitted to the vibrator, relatively large noise is induced in the signal output from the vibrator. Vibration transmitted through a car chassis to a vibratory gyroscope usually includes various frequency components, and the frequency component of the detuned frequency causes the above noise. Therefore, it is necessary to reduce such xe2x80x9cvibration noisexe2x80x9d induced by an external vibration including the frequency component of the detuned frequency.
The present invention aims to reduce xe2x80x9cvibration noisexe2x80x9d which is included in the output of a vibrator used in vibratory gyroscopes for detecting the turning angular rate, when the gyroscopes are subjected to external vibrations including the frequency component of the detuned frequency of the vibrator.
One aspect of the present invention provides a vibratory gyroscope for detecting a turning angular rate;
wherein the gyroscope comprises a vibrator made out of a piezoelectric material, the vibrator has a driving vibration mode in which the vibrator is electrically excited and another detection vibration mode for detecting Coriolis vibration occurring in the vibrator when the vibrator is rotated around a detection axis at a turning angular rate, and the vibrator has a surface including a region where stress induced in the vibrator in the detection vibration mode is in a range of a local maximum, the gyroscope further comprising a damper for reducing vibration sensitivity to external vibrations applied externally on the vibrator, the damper being made of a polymer and provided on at least said region.
In this embodiment, the vibrator has a region which does not substantially vibrate in the driving vibration mode and substantially vibrates in the detection vibration mode and a detecting electrode provided on the region. In the driving vibration mode, the amplitude of vibration in the region may preferably be not more than 0.01 of the maximum amplitude of vibration in the vibrator.
Another aspect of the present invention also provides a vibratory gyroscope for detecting a turning angular rate;
the gyroscope comprising a vibrator made of a piezoelectric material, the vibrator comprising one or more driving parts for electrically exciting a driving-mode vibration in the vibrator, one or more detecting parts provided separately from the driving part for detecting a Coriolis vibration occurring in the vibrator when the vibrator is rotated, and a damper for reducing vibration sensitivity to external vibrations applied externally on the vibrator, the damper being made of a polymer and is provided on a surface of the detecting part. Preferably, the damper is not provided on the driving part.
The xe2x80x9cdriving partxe2x80x9d is defined as a part on which a driving means such as a driving electrode is provided. The xe2x80x9cdetection partxe2x80x9d is defined as a part on which a detection means such as a detection electrode is provided. In this aspect, the detection part does not substantially vibrate in the driving vibration mode. Alternatively, in the driving vibration mode, the amplitude in detection part may preferably be not more than 0.01 of the maximum amplitude of vibration in the vibrator.
The present invention also provides a method for manufacturing a vibratory gyroscope for detecting a turning angular rate, the gyroscope comprising a vibrator which has a driving vibration mode for electrically vibrating the vibrator and has a detection vibration mode for detecting Coriolis vibration occurring in the vibrator due to the rotation of the vibrator, the method comprising the steps of;
computing each ratio of each stress at each point of the vibrator to a maximum stress in the whole vibrator by means of a characteristic mode analysis by the finite element method, for the case of the detection vibration mode, determining a region where the ratio is in a range of a local maximum, and providing a damper made of a polymer on the region.
The present inventor succeeded to substantially reduce the above vibration noise included in signal output from a vibrator having a detecting arm, when external vibration including the frequency component of the detuned frequency is transmitted to the vibrator, by providing a damper made of a polymer on the detecting arm.
When the vibrator has a detecting part or parts and a base part, the damper may be provided on either of its main faces and side faces, or on both the main faces, or on either of the main faces and on the side faces, or on both the main faces and on both the side faces. When the detecting part has an elongated shape, and has a root portion extending onto the base part, the damper may preferably be provided in a position which distance from the root is within a half of the length of the detecting part. The damper provided on or near the end portion of the detection part may increase the temperature drift of the gyroscope.
In the above publication xe2x80x9cSensors and Actuatorsxe2x80x9d, the authors tried to reduce the vibration noise detected from the gyroscope due to the beat, by increasing the detuned frequency sufficiently larger than the cut off frequency of the low pass filter installed in the gyroscope. However, such method described in the publication does not reduce the induction of the vibration noise in the vibrator due to an external vibration, but reduces such vibration noise component by means of an electric filter. Because the method increases the detuned frequency, the sensitivity of the gyroscope and its signal/noise ratio decrease in an inversely proportional manner to the detuned frequency.
When a damper is provided on the surface of a vibrator, the damper may preferably be provided on a region where stress induced in the vibrator in the detection vibration mode is in a range of a local maximum oh the surface of the vibrator. Such a region includes and is not necessarily identical with the region where stress induced in the vibrator is in a range of the maximum, which does not necessarily mean local maximum. However, assuming a maximum stress in the vibrator in the detection vibration mode as 1.0, the damper may preferably be provided on a region where its minimum stress is not less than 0.01 and its maximum stress is not less than 0.7, more preferably not less than 0.8.
The damper may preferably be provided, on the surface of the vibrator, on a region where the stress in the detection vibration mode is large, thus maximizing the effect of reducing the vibration noise due to external vibration including the frequency component of the detuned frequency. Moreover, the damper may preferably cover the whole of the region where the stress in the detection vibration mode is large, thus reducing the fluctuation inevitably caused during the actual manufacturing process of the damper.
The damper may preferably be provided, on the surface of the vibrator, on a region where the ratio of the stress induced in the vibrator to its maximum value in the driving vibration mode is not more than 0.1, not to increase the driving impedance of the vibrator. A larger driving impedance needs a higher driving voltage for the driving circuit, and consumes more power in the vibratory gyroscope.
Preferably, the damper may not be provided on a region in which each amplitude of vibration of each point is not less than 0.1 of the maximum amplitude of vibration in the vibrator in the driving vibration mode. The damper provided on a region, in which amplitude of vibration is large in the driving vibration mode, increases the driving impedance of the vibrator, the voltage necessary for driving the vibrator and thus the overall consumed electrical power of the gyroscope. Moreover, such damper also increases the temperature drift of the gyroscope because the viscoelasticity of the damper is changed over its temperature, thus affecting the output signal. Such adverse effects of the temperature drift may be more substantial when the vibrator is made of a piezoelectric single crystal such as quartz.
When the vibrator is plate-shaped with two main faces and side faces, and when its detection mode vibration is parallel with the main faces, the region with maximum stress induced in the vibrator in the detection vibration mode is often located on the side face of the faces of the detecting part. In this case, the damper may preferably be provided on such side face of the faces of the detecting part. However, when the damper is not provided on the region with maximum stress on the side face or faces and provided on either or both of regions, adjacent to the region with maximum stress on the side face, of the two main faces, the above inventive effects may also be obtained. The damper may preferably be provided on the detecting electrode on the detecting part. Most preferably, the damper is provided only on the detecting part and is not provided on any other portions of the vibrator.
The inventive vibrator may be formed out of a permanent elastic alloy such as elinver. In this case, a polycrystalline piezoelectric element is formed on the vibrator. The inventive vibrator may preferably be made of a piezoelectric single crystal. In this case, a pair of driving electrodes, for exciting vibration in the driving vibration mode, and a pair of detecting electrodes, for detecting Coriolis vibration occurring in the vibrator due to the rotation of the vibrator, are provided on the vibrator. Since a piezoelectric single crystal usually has an extremely low viscosity, the inventive polymeric damper may be particularly effective. Such a single crystal includes quartz and single crystals of LiTaO3, LiNbO3 and LiTaNbO3.
The damper may be formed of a viscoelastic material with a small temperature variation in its dynamic viscoelasticity between xe2x88x9240xc2x0 C. to +85xc2x0 C., in which the vibratory gyroscope Is usually used. The ratio of the maximum to the minimum of the dynamic viscoelasticity may preferably be not more than 3.0 between xe2x88x9240xc2x0 C. to +85xc2x0 C. Such viscoelastic material includes a silicone rubber, a synthetic rubber such as ethylene-propylene rubber, butyl rubber and urethane rubber, a fluoride resin such as xe2x80x9cTeflonxe2x80x9d and ethylene-tetrafluoride resin, vinyl chloride resin, xe2x80x9cnylonxe2x80x9d and polyethylene resin. The viscoelastic material may preferably has a dynamic modulus of 102 to 1010 Pa and a dynamic loss of 101 to 108 Pa. The thickness of the damper may be generally adjusted so as to be inversely proportional to the dynamic viscoelasticity.
The damper may be formed by means of any method. For example, the damper may be a cured coating film formed on the vibrator by means of coating, potting, or spray coating. For example, various silicone adhesive agents of, for example, alcohol-liberating, acetone-liberating, oxime-liberating, acetic acid-liberating, or addition reaction types may be potted and adhered on the vibrator by means of a dispenser. When liquid type material is coated or potted on the vibrator, such liquid type material may preferably have a viscosity of not more than 100 Paxc2x7s to readily cover a larger area and to produce a coated film with a uniform thickness. A sheet or plate shaped material may be adhered on the vibrator to provide the inventive damper.
The inventive vibratory gyroscope may have external members contacting the vibrator. Such members may preferably be located in positions symmetrical with respect to the center of gravity GO of the vibrator. For example, leads, for electrically connecting electrodes on the vibrator and an external electrical circuit, may preferably be located in positions symmetrical with respect to the center of gravity GO.
The vibrator may be fixed to a base, such as a can package, by means of a supporting member. In this case, a buffer member, made of a material (such as a polymer) with an elasticity lower than that of the material constituting the vibrator, may be inserted between the vibrator and the supporting member. The vibrator may vibrate in torsion and deflection vibration modes with the supporting member as its fulcrum, as a result of the deformation of the buffer member. The elasticity, dimensions and shape of the buffer member may be adjusted so that the two vibration modes have their eigenfrequencies of 200 Hz to 2 kHz.
The file of this patent contains at least one drawing executed in color. Copies of this patent with color drawing(s) will be provided by the Patent and Trademark Office upon request and payment of the necessary fee.
For a better understanding of the invention, reference is made to the attached drawings, wherein:
FIG. 1 is a plane view schematically showing a vibrator for a vibratory gyroscope of the present invention,
FIGS. 2(a), 2(b) and 2(c) are side views showing a detecting part 16 of the vibrator, detecting electrodes 14A, 14B and 14D and a damper 17A,
FIG. 3 is a colored map showing the distribution of stress in the vibrator of FIG. 1 in its detection vibration mode,
FIG. 4 is a colored map showing the ratio of the vibration amplitude at each point, in the vibrator of FIG. 1, to its maximum amplitude in the whole vibrator in its driving vibration mode,
FIG. 5 is a colored map showing the ratio of the vibration amplitude at each point, in the vibrator of FIG. 1, to its maximum amplitude in the whole vibrator in its detection vibration mode,
FIG. 6(a) is a plain view illustrating the distribution of stress in one main face 10 in the detecting part 16 of the vibrator and the location of the damper 17A,
FIG. 6(b) is a plain view illustrating the distribution of stress in one side face 11 in the detecting part 16 and the location of the damper 17B,
FIGS. 7(a), 7(b), 7(c) and 7(d) are cross sectional views schematically showing the examples of structures supporting the inventive vibrator,
FIG. 8 is a graph showing the relationship between the frequency of external vibration and the vibration sensitivity of the vibratory gyroscope, in the example 1
FIG. 9 is a graph showing the relationship between the frequency of external vibration and the vibration sensitivity of the vibratory gyroscope, in the comparative example 1,
FIG. 10 is a colored map showing the distribution of stress in the detecting part 16 in the detection vibration mode of the vibrator of FIG. 1,
FIG. 11 is a line diagram illustrating the distribution of stress shown in FIG. 10, and
FIG. 12 is a graph showing the relationship between the distance of each point along the center line of the main face 10 from the root of the detecting part 16, and the ratio of stress at the each point to the maximum stress in the whole vibrator.